Methods for near surface work function engineering

ABSTRACT

Methods for adjusting a work function of a structure in a substrate leverage near surface doping. In some embodiments, a method for adjusting a work function of a structure in a substrate may include growing an epitaxial layer on surfaces of the structure to form a homogeneous passivation region as part of a substrate material of the substrate and performing a dopant diffusion process to further embed the dopants into surfaces of the structure to adjust a work function of the structure, wherein the dopant diffusion process is performed at less than approximately 450 degrees Celsius.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to International Patent Application No.PCT/US2022/013044, filed Jan. 20, 2022, and entitled “METHODS FOR NEARSURFACE WORK FUNCTION ENGINEERING”, the contents of which are herebyincorporated by reference in its entirety.

FIELD

Embodiments of the present principles generally relate to semiconductorprocessing of semiconductor substrates.

BACKGROUND

Structures, such as trenches, are often formed on substrates as part ofconstructing a semiconductor device. Bulk processing and hightemperatures are typically used to alter the work function of thestructures. However, the inventor has found that during such processes,the high temperatures may damage surrounding devices and also causelarge charge gradients areas to be formed in the structures, reducingthe area available for other device layers, causing a substantialdecrease in performance.

Accordingly, the inventor has provided improved processes for nearsurface work function engineering that substantially increasesperformance and quantum efficiencies of structures.

SUMMARY

Methods and structures for improved near surface work functionengineering are provided herein.

In some embodiments, a method of adjusting a work function of astructure on a substrate may comprise growing an epitaxial layer onsurfaces of the structure to form a homogeneous passivation region withdopants as part of a substrate material of the substrate and performinga dopant diffusion process to further embed the dopants into surfaces ofthe structure to adjust the work function of the structure, wherein thedopant diffusion process is performed at less than approximately 450degrees Celsius.

In some embodiments, the method may further include wherein the dopantdiffusion process yields a charge in surfaces of the structure of up toplus or minus approximately 3e18/cm³ to approximately 3e21/cm³, whereinthe dopants are P-type or N-type, wherein the dopant diffusion processforms an abrupt junction profile, wherein the method is performed in aback-end-of-line (BEOL) process, wherein the epitaxial layer is formedof single crystals, wherein the epitaxial layer is a non-crystal layer,growing the epitaxial layer using a low temperature process of less thanapproximately 450 degrees Celsius, forming the structure using an etchprocess, forming an oxide layer on surfaces of the structure with a dryoxide process at a temperature of less than approximately 450 degreesCelsius with a controllable oxidation thickness of approximately 1 nm toapproximately 15 nm, and selectively removing the oxide layer fromsurfaces of the structure prior to growing the epitaxial layer, whereinthe dry oxide process is performed in a plasma oxidation chamber, and/oretching the structure into the substrate to a high aspect ratio ofgreater than approximately 75:1.

In some embodiments, a method of adjusting a work function of astructure on a substrate may comprise forming a non-crystal materiallayer with a low temperature process on surfaces of the structure toform a homogeneous passivation region with dopants as part of asubstrate material of the substrate and performing a dopant diffusionprocess to further embed the dopants into surfaces of the structure toadjust the work function of the structure and to form an oxide layerfrom the non-crystal layer, wherein the dopant diffusion process isperformed at less than approximately 450 degrees Celsius and forms acharge layer with an abrupt junction profile.

In some embodiments, the method may further include wherein the dopantdiffusion process yields a charge in surfaces of the structure of up toplus or minus approximately 3e18/cm³ to approximately 3e21/cm³, whereinthe dopants are P-type or N-type, wherein the method is performed in aback-end-of-line (BEOL) process, wherein the dopant diffusion process isa condensation-based process that embeds the dopants and forms the oxidelayer, and/or wherein the non-crystal material layer is formed at atemperature of less than 450 degrees Celsius and the dopant diffusionprocess is performed at a temperature of less than 450 degrees Celsius.

In some embodiments, a non-transitory, computer readable medium havinginstructions stored thereon that, when executed, cause a method foradjusting a work function of a structure in a substrate to be performed,the method may comprise growing an epitaxial layer on surfaces of thestructure to form a homogeneous passivation region as part of asubstrate material of the substrate and performing a dopant diffusionprocess to further embed the dopants into surfaces of the structure toadjust the work function of the structure, wherein the dopant diffusionprocess is performed at less than approximately 450 degrees Celsius.

In some embodiments, the method of the non-transitory, computer readablemedium may further include wherein the dopant diffusion process yields acharge in surfaces of the structure of up to plus or minus approximately3e18/cm³ to approximately 3e21/cm³ and forms an abrupt junction profileand/or growing the epitaxial layer using a low temperature process ofless than approximately 450 degrees Celsius.

Other and further embodiments are disclosed below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present principles, briefly summarized above anddiscussed in greater detail below, can be understood by reference to theillustrative embodiments of the principles depicted in the appendeddrawings. However, the appended drawings illustrate only typicalembodiments of the principles and are thus not to be considered limitingof scope, for the principles may admit to other equally effectiveembodiments.

FIG. 1 is a method of adjusting a work function of surfaces of a trenchstructure in a substrate in accordance with some embodiments of thepresent principles.

FIG. 2A depicts a cross-sectional view of a trench structure after anetching process in accordance with some embodiments of the presentprinciples.

FIG. 2B depicts a cross-sectional view of a trench structure after a dryoxide process in accordance with some embodiments of the presentprinciples.

FIG. 2C depicts a cross-sectional view of a trench structure after aselective oxide removal process in accordance with some embodiments ofthe present principles.

FIG. 2D depicts a cross-sectional view of a trench structure after aforming passivation region in accordance with some embodiments of thepresent principles.

FIG. 2E depicts a cross-sectional view of a trench structure after anoptional gas doping of the passivation region in accordance with someembodiments of the present principles.

FIG. 2F depicts a cross-sectional view of a trench structure after awork function is adjusted in accordance with some embodiments of thepresent principles.

FIG. 2G depicts a cross-sectional view of a trench structure after analternative method of a dopant diffusion process in accordance with someembodiments of the present principles.

FIG. 3 depicts an integrated tool in accordance with some embodiments ofthe present principles.

FIG. 4A depicts a first process of a second approach in accordance withsome embodiments of the present principles.

FIG. 4B depicts a second process of a second approach in accordance withsome embodiments of the present principles.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. The figures are not drawn to scale and may be simplifiedfor clarity. Elements and features of one embodiment may be beneficiallyincorporated in other embodiments without further recitation.

DETAILED DESCRIPTION

The methods provide a high-performance near surface work functionengineering solution that dramatically increases charge manipulationcapabilities in surfaces of structures such as trenches and the like.The methods provide innovative dopant and junction formation withprecise profile control without the thermal budget constraints found intraditional processes. The techniques enable abrupt junction formationof charges near surfaces with ultra-high activated doping withoutcrystal damage. In addition, the techniques allow use inback-end-of-line (BEOL) processes without fear of thermal damage toexisting structures on a substrate. The methods are also compatible withsurfaces of high aspect ratio structures of greater than 100:1.

Although a trench is used for the sake of brevity in the followingexamples, other structures may benefit from the methods of the presentprinciples and, therefore, the use of a trench in the examples is notmeant to be limiting. For example, the techniques of the presentprinciples may also be used for planar structures as well. FIG. 1 is amethod 100 of adjusting a work function of a trench structure 204 in asubstrate 202 in accordance with some embodiments. In block 102, in someembodiments, an etching process, for example, forms a trench structure204 into the substrate 202 as depicted in view 200A of FIG. 2A. Theetching process typically uses a hardmask layer 212 that protects areasfrom the etching process. In some embodiments, the aspect ratio of thetrench structure 204 is at least approximately 50:1. In someembodiments, the aspect ratio of the trench structure 204 is at leastapproximately 75:1. In some embodiments, the aspect ratio of the trenchstructure 204 is at least approximately 100:1. As a side effect of theetching process, damage 208 occurs to the surfaces 206 of the trenchstructure 204. The damage 208 may include crystal damage of thesubstrate material, contaminants or residue from the etching process,and/or dangling bonds of the substrate material and the like. Thehardmask layer 212 is removed after the etching process is completed.

To further prepare the trench structure 204, in some embodiments, anoxide layer 216 is formed on the substrate 202 using a dry oxidationprocess. As depicted in a view 200B of FIG. 2B, the field 214 or topsurfaces of the substrate 202 and the surfaces 206 of the trenchstructure 204 undergo a dry oxidation process to form an oxide layer 216that partially consumes the material of the substrate 202 includingdamaged portions. The dry oxidation process can be performed attemperatures of less than 450 degrees Celsius and produce lesscontamination and residue when compared to wet oxidation processes. Inaddition, dry oxidation processes can be used in substantially higheraspect ratio structures (e.g., greater than 100:1 aspect ratios) thanwet oxidation (e.g., less than 50:1 aspect ratios). In some embodiments,the dry oxidation process is performed with a plasma oxidation chamberwith or without a remote plasma source. The dry oxidation processfacilitates in embedding oxygen into the surfaces 206 of the trenchstructure 204 to repair damage to the surfaces 206 and reduce stressinduced leakage current (SILC) and interface trap densities (D_(it)).

The dry oxidation process can also be controlled to provide differentthicknesses of the oxide layer 216. Parameters such as exposure time,plasma density, temperature and the like can facilitate in determiningan oxidation rate. The thickness is then controlled by the duration ofthe dry oxidation process. In conventional methods such as wetoxidation, the oxidizing process is self-limiting (wet oxidation processis self-terminating) and oxide layer thicknesses cannot be adjusted. Wetoxidation typically stops at 1 nm to 2 nm of thickness at the saturationpoint. Dry oxidation does not have a saturation point and is notself-limiting allowing any level of thickness to be obtained. In someembodiments, the dry oxidation processes can achieve conformality in thetrench structure 204 of greater than 95% for trenches with an aspectratio of greater than 100:1, enabling scaling of trench isolationstructures using the present principles. In some embodiments, the oxidelayer 216 is selectively removed from the surfaces 206 of the trenchstructure 204 and the field 214 of the substrate 202 as depicted in aview 200C of FIG. 2C. In some embodiments, plasma-based chambers can beused to selectively remove the oxide layer 216 with selectivity ratiosof, for example, greater than 50:1 (e.g., oxide over Si or SiGe). Theselective removal of the oxide layer 216 removes all of the oxide layer216 without damaging any of the underlying material of the substrate 202or creating contaminants/residue, leaving damage free surfaces of thetrench structure 204.

In block 104, a passivation region 210 is formed on the surfaces 206 ofthe trench structure 204 and the field 214 of the substrate 202 asdepicted in a view 200D of FIG. 2D. In some embodiments, the passivationregion 210 is formed of homogeneous material similar to the material ofthe substrate 202. The passivation region 210 is formed by singlecrystals of the material with a species of dopants 222 (P-type speciesshown but not meant to be limiting) incorporated to form a positivecharge or a negative charge (e.g., silicon doped with boron, gallium,phosphor, arsenic, etc.) on the substrate 202 and can be fabricated bytwo approaches. The first approach is with a low temperature epitaxialgrowth process using temperatures of less than approximately 450 degreesCelsius. The second approach is a two-part process in which the firstpart is to form a non-crystal doped material layer 402 on the field 214as depicted in a view 400A of FIG. 4A followed by a second partincluding a material engineering process 404 such as oxidation orthermal treatment to drive dopants from the non-crystal doped materiallayer 402 into the surface of the substrate 202 as depicted in a view400B of FIG. 4B. The non-crystal doped material layer 402 becomes anoxide layer 402A. Thus, the passivation region 210 can be formed assingle crystal material under the field 214 or as a non-crystal materiallayer under the field 214.

In essence, the passivation region 210 becomes part of or an extensionof the substrate material and does not form an interface between thepassivation region 210 and the substrate 202, eliminating an interfacecommonly found in conventional processes that form a heterogeneouspassivation region. The passivation region 210 also does not have anyoptical penalty as photons pass through the passivation region 210 asthe photons would through the material of the substrate 202 without anydegradation or change in path (refraction). The growth of the chargelayer of the passivation region 210 also serves in repairing danglingbonds of the surfaces 206 of the trench structure 204 caused duringtrench etching processes. In some embodiments, the portion of thepassivation region 210 on the field 214 of the substrate 202 may beremoved, leaving only the portion of the passivation region 210 in thetrench structure 204. In some embodiments, the passivation region 210can incorporate species used for engineering material properties such asenergy band, light sensitivity, etc. and can have gradient compositiontransition from the substrate 202 to the passivation region 210 to avoidthe interface formation.

Optionally, in some embodiments, the passivation region 210 may beformed without dopants which are then introduced via gases 220 asdepicted in a view 200E of FIG. 2E. In further optional embodiments, thepassivation region 210 may be formed with dopants and then enhanced withadditional dopants by exposure to gases. The substrate 202 is exposed togases 220 including dopants 222B that embed in the passivation region210 on the field 214 of the substrate 202 and the surfaces 206 of thetrench structure 204. The dopants 222B (as shown) in FIG. 2E have apositive charge for the sake of brevity and is not meant to be limiting.The dopants 222B may also have a negative charge (not shown). Gases 220can also incorporate species such as Ge, carbon, for materialengineering purposes on composition, energy band, light sensitivity,etc. The density of the dopants 222B and/or the type of the dopants 222Bmay be adjusted to provide a given plus or minus charge level asrequired for the trench structure 204.

In block 106, a work function of the passivation region 210 can beadjusted as depicted in a view 200F of FIG. 2F. In some embodiments, acharge layer 226 is formed in-situ during the formation of thepassivation region 210. A work function of the trench structure 204 isalso adjusted by varying dopant densities, dopant types, and dopantdepths as follows. The density of the dopants 222 and/or the type of thedopants 222 may be adjusted to provide a given plus or minus chargelevel as required for the trench structure 204 and a given workfunction. The dopants 222 and/or species different from substrate 202are further embedded into the material of the substrate 202 by a dopantdiffusion process 224 to further adjust the work function of the trenchstructure 204. A higher work function near a surface can facilitate orincrease the carrier mobility inside of an adjacent structure such as apixel structure and reduce the sensitivity to the surface recombination.The above techniques allow for substantially flexibility in engineeringthe work function through dopant depth, density, and type adjustmentswhile increasing adjacent structure area by using abrupt junctionprofiles.

In some embodiments, the dopant diffusion process 224 may be amillisecond anneal process that uses high power and high temperaturelasers (e.g., 700 degrees Celsius to 900 degrees Celsius) that arepulsed in the millisecond range to anneal the substrate 202 withoutheating the substrate to high temperatures. The dopant diffusion process224 embeds the charge layer 226 in the material of the substrate 202adjacent to the passivation region. In some embodiments, a plasmaoxidation process may be performed on the substrate 202 as the dopantdiffusion process 224 to drive the dopants 222 and the charge layer 226further into the substrate 202 than can be obtained with the millisecondanneal process. In some embodiments, a combination of the millisecondanneal process and the plasma oxidation process may be used as thedopant diffusion process 224.

The dopant diffusion process 224 yields an abrupt charge boundary orabrupt junction profile or abrupt composition transition in the materialof the substrate 202 that increases the effective area of an adjacentstructure (e.g., a pixel structure area, etc.) as opposed toconventional techniques that form a gradient charge region that requiresmore area to be used adjacent to the trench structure, reducingperformance of adjacent structures. In some embodiments, a charge layermay have a charge formation of up to plus or minus approximately3e18/cm³ to approximately 3e21/cm³ depending on permitted temperatureand pressure (higher temperature and/or higher pressure yield highercharge densities). The charge layer may be formed homogeneously orheterogeneously. The charge layer of the present principles has near100% dopant activation as processed without the need of any postactivation treatment. In some embodiments, the above processes may beperformed without an air break to prevent surface impurities,contaminants, and/or particle generation. In some embodiments, thegrowth of the charge layer of the passivation region 210 can be oxidizedby condensation to both drive the dopants further into the material ofthe substrate 202 and form an oxide layer 230 as depicted in a view 200Gof FIG. 2G. In effect, the passivation region 210 produces a liner layer218 and embedded charge layer without an anneal process such as themillisecond anneal process discussed above. In addition, the chargelayer can be non-crystal rather than single crystal growth.

The methods described herein may be performed in individual processchambers that may be provided in a standalone configuration or as partof a cluster tool, for example, an integrated tool 300 (i.e., clustertool) described below with respect to FIG. 3 . The advantage of using anintegrated tool 300 is that there is no vacuum break and, therefore, norequirement to degas and pre-clean a substrate before treatment in achamber. For example, in some embodiments the inventive methodsdiscussed above may advantageously be performed in an integrated toolsuch that there are limited or no vacuum breaks between processes,limiting or preventing contamination of the substrate. The integratedtool 300 includes a vacuum-tight processing platform 301, a factoryinterface 304, and a system controller 302. The processing platform 301comprises multiple processing chambers, such as 314A, 313B, 314C, 314D,314E, and 314F operatively coupled to a vacuum substrate transferchamber (transfer chambers 303A, 303B). The factory interface 304 isoperatively coupled to the transfer chamber 303A by one or more loadlock chambers (two load lock chambers, such as 306A and 306B shown inFIG. 3 ).

In some embodiments, the factory interface 304 comprises at least onedocking station 307, at least one factory interface robot 338 tofacilitate the transfer of the semiconductor substrates. The dockingstation 307 is configured to accept one or more front opening unifiedpod (FOUP). Four FOUPS, such as 305A, 305B, 305C, and 305D are shown inthe embodiment of FIG. 3 . The factory interface robot 338 is configuredto transfer the substrates from the factory interface 304 to theprocessing platform 301 through the load lock chambers, such as 306A and306B. Each of the load lock chambers 306A and 306B have a first portcoupled to the factory interface 304 and a second port coupled to thetransfer chamber 303A. The load lock chamber 306A and 306B are coupledto a pressure control system (not shown) which pumps down and vents theload lock chambers 306A and 306B to facilitate passing the substratesbetween the vacuum environment of the transfer chamber 303A and thesubstantially ambient (e.g., atmospheric) environment of the factoryinterface 304. The transfer chambers 303A, 303B have vacuum robots 342A,342B disposed in the respective transfer chambers 303A, 303B. The vacuumrobot 342A is capable of transferring substrates 321 between the loadlock chamber 306A, 306B, the processing chambers 314A and 314F and acooldown station 340 or a pre-clean station 342. The vacuum robot 342Bis capable of transferring substrates 321 between the cooldown station340 or pre-clean station 342 and the processing chambers 314B, 314C,314D, and 314E.

In some embodiments, the processing chambers 314A, 314B, 314C, 314D,314E, and 314F are coupled to the transfer chambers 303A, 303B. Theprocessing chambers 314A, 314B, 314C, 314D, 314E, and 314F may comprise,for example, an atomic layer deposition (ALD) process chamber, aphysical vapor deposition (PVD) process chamber, chemical vapordeposition (CVD) chambers, annealing chambers, or the like. The chambersmay include any chambers suitable to perform all or portions of themethods described herein, as discussed above, such as a dry oxideremoval chamber or pre-clean chamber and an epitaxial growth chamberalong with etching and deposition chambers. In some embodiments, one ormore optional service chambers (shown as 316A and 316B) may be coupledto the transfer chamber 303A. The service chambers 316A and 316B may beconfigured to perform other substrate processes, such as degassing,orientation, substrate metrology, cool down and the like.

The system controller 302 controls the operation of the tool 300 using adirect control of the process chambers 314A, 314B, 314C, 314D, 314E, and314F or alternatively, by controlling the computers (or controllers)associated with the process chambers 314A, 314B, 314C, 314D, 314E, and314F and the tool 300. In operation, the system controller 302 enablesdata collection and feedback from the respective chambers and systems tooptimize performance of the tool 300. The system controller 302generally includes a Central Processing Unit (CPU) 330, a memory 334,and a support circuit 332. The CPU 330 may be any form of ageneral-purpose computer processor that can be used in an industrialsetting. The support circuit 332 is conventionally coupled to the CPU330 and may comprise a cache, clock circuits, input/output subsystems,power supplies, and the like. Software routines, such as a method asdescribed above may be stored in the memory 334 and, when executed bythe CPU 330, transform the CPU 330 into a specific purpose computer(system controller) 302. The software routines may also be stored and/orexecuted by a second controller (not shown) that is located remotelyfrom the tool 300.

Embodiments in accordance with the present principles may be implementedin hardware, firmware, software, or any combination thereof. Embodimentsmay also be implemented as instructions stored using one or morecomputer readable media, which may be read and executed by one or moreprocessors. A computer readable medium may include any mechanism forstoring or transmitting information in a form readable by a machine(e.g., a computing platform or a “virtual machine” running on one ormore computing platforms). For example, a computer readable medium mayinclude any suitable form of volatile or non-volatile memory. In someembodiments, the computer readable media may include a non-transitorycomputer readable medium.

While the foregoing is directed to embodiments of the presentprinciples, other and further embodiments of the principles may bedevised without departing from the basic scope thereof.

The invention claimed is:
 1. A method of adjusting a work function of astructure on a substrate, comprising: growing an epitaxial layer onsurfaces of the structure to form a homogeneous passivation region withdopants as part of a substrate material of the substrate; and performinga dopant diffusion process to further embed the dopants into surfaces ofthe structure to adjust the work function of the structure, wherein thedopant diffusion process is performed at less than approximately 450degrees Celsius.
 2. The method of claim 1, wherein the dopant diffusionprocess yields a charge in surfaces of the structure of up to plus orminus approximately 3e18/cm³ to approximately 3e21/cm³.
 3. The method ofclaim 1, wherein the dopants are P-type or N-type.
 4. The method ofclaim 1, wherein the dopant diffusion process forms an abrupt junctionprofile.
 5. The method of claim 1 is performed in a back-end-of-line(BEOL) process.
 6. The method of claim 1, wherein the epitaxial layer isformed of single crystals.
 7. The method of claim 1, wherein theepitaxial layer is a non-crystal layer.
 8. The method of claim 1,further comprising: growing the epitaxial layer using a low temperatureprocess of less than approximately 450 degrees Celsius.
 9. The method ofclaim 1, further comprising: forming the structure using an etchprocess; forming an oxide layer on surfaces of the structure with a dryoxide process at a temperature of less than approximately 450 degreesCelsius with a controllable oxidation thickness of approximately 1 nm toapproximately 15 nm; and selectively removing the oxide layer fromsurfaces of the structure prior to growing the epitaxial layer.
 10. Themethod of claim 9, wherein the dry oxide process is performed in aplasma oxidation chamber.
 11. The method of claim 9, further comprising:etching the structure into the substrate to a high aspect ratio ofgreater than approximately 75:1.
 12. A method of adjusting a workfunction of a structure on a substrate, comprising: forming anon-crystal material layer with a low temperature process on surfaces ofthe structure to form a homogeneous passivation region with dopants aspart of a substrate material of the substrate; and performing a dopantdiffusion process to further embed the dopants into surfaces of thestructure to adjust the work function of the structure and to form anoxide layer from the non-crystal material layer, wherein the dopantdiffusion process is performed at less than approximately 450 degreesCelsius and forms a charge layer with an abrupt junction profile. 13.The method of claim 12, wherein the dopant diffusion process yields acharge in surfaces of the structure of up to plus or minus approximately3e18/cm³ to approximately 3e21/cm³.
 14. The method of claim 12, whereinthe dopants are P-type or N-type.
 15. The method of claim 12 isperformed in a back-end-of-line (BEOL) process.
 16. The method of claim12, wherein the dopant diffusion process is a condensation-based processthat embeds the dopants and forms the oxide layer.
 17. The method ofclaim 12, wherein the non-crystal material layer is formed at atemperature of less than 450 degrees Celsius and the dopant diffusionprocess is performed at a temperature of less than 450 degrees Celsius.18. A non-transitory, computer readable medium having instructionsstored thereon that, when executed, cause a method for adjusting a workfunction of a structure in a substrate to be performed, the methodcomprising: growing an epitaxial layer on surfaces of the structure toform a homogeneous passivation region as part of a substrate material ofthe substrate; and performing a dopant diffusion process to furtherembed the dopants into surfaces of the structure to adjust the workfunction of the structure, wherein the dopant diffusion process isperformed at less than approximately 450 degrees Celsius.
 19. Thenon-transitory, computer readable medium of claim 18, wherein the dopantdiffusion process yields a charge in surfaces of the structure of up toplus or minus approximately 3e18/cm³ to approximately 3e21/cm³ and formsan abrupt junction profile.
 20. The non-transitory, computer readablemedium of claim 18, further comprising: growing the epitaxial layerusing a low temperature process of less than approximately 450 degreesCelsius.